1. Field of the Invention
The present invention relates to removal of intervertebral discs and, more particularly, to apparatus and methods for removal of the nucleus pulposus of an intervertebral disc.
2. Description of the Related Art
The spine is made up of twenty-four bony vertebrae, each separated by a disc that both connects the vertebrae and provides cushioning between them. The lumbar portion of the spine has five vertebrae, the last of which connects to the sacrum. The disc is comprised of the annulus, which is a tough, layered ligamentous ring of tissue that connects the vertebrae together, and the nucleus, a gelatinous material that absorbs water and is fed through the endplates of the vertebrae. In a healthy disc, the nucleus is pressurized within the annulus much like the air is pressurized within an automobile tire.
Degenerative disc disease (DDD) is a condition that affects both structures of the disc, and is usually thought of as a cascade of events. In general, DDD is characterized by a weakening of the annulus and permanent changes in the nucleus, and may be caused by extreme stresses on the spine, poor tone of the surrounding muscles, poor nutrition, smoking, or other factors. In DDD, the nutrient flow to the nucleus is disrupted and the nucleus loses water content. As the nucleus dehydrates it loses pressure, resulting in a loss of disc height and a loss in the stability of that segment of the spine. In the lumbar spine, as the degenerative cascade continues, the annulus may bulge and press on a nerve root, causing sciatica (leg pain) among other problems. The loss of disc height can also result in leg pain by reducing the size of the opening for the nerve root through the bony structures of the spine. As the disc loses height, the layers of the annulus can begin to separate, irritating the nerves in the annulus and resulting in back pain.
Surgical treatment for early DDD, where the pain is primarily leg pain, is usually a discectomy where some the nucleus material is removed to reduce the bulging of the disc and the pressure on the nerve root. For more severe cases of DDD, where the disc has completely collapsed and/or where a discectomy did not have long-term success, the surgical treatment standard of care is fusion of the vertebrae through the use of plates, rods, pedicle screws, and interbody fusion devices. The trend in fusion surgery is an increasing use of less invasive techniques, which reduce post-operative pain and patient recovery times by reducing the amount of tissue disruption during the surgery. Recently, surgeons and industry have been looking for ways to interrupt the degenerative cascade earlier in the disease process, and for methods that retain motion at the affected disc in patients with more advanced disease. The field of spinal arthroplasty represents a significant emerging market in spinal surgery, and includes devices known as total disc replacements (TDR) and partial disc replacements (PDR) where only the nucleus of the disc is replaced by a prosthetic device.
Surgical treatment for early stage disease that involves primarily leg pain as a result of a herniated disc is currently limited to a simple discectomy, where a small portion of the disc nucleus is removed to reduce pressure on the nerve root, the cause of the leg pain. While this procedure is usually immediately successful, it offers no means to prevent further degeneration, and a subsequent herniation requiring surgery will occur in about 15% of these patients.
The method currently used for most intervertebral fusion procedures involves placing an interbody fusion device in the disc nucleus cavity, which supports and stabilizes the anterior column of the spine. Small pieces of the patient's own bone, taken either from the bone removed from the spine during surgical access or from a donor site such as the iliac crest, are packed in and around the fusion device to speed the subsequent bone growth process. It has been well-established that any nucleus material remaining in the disc space following the fusion surgery will likely interfere with the fusion process by acting both as a mechanical and biological barrier to bone growth.
Current designs for nucleus replacement devices are typically not attached to the nucleus or vertebra, and are free to move within the nucleus cavity. Much like the healthy nucleus, these devices are subjected to the high forces and the twisting and bending motions that must be endured by the spinal structures, and some device movement is expected. Current PDR devices have a known complication of excessive device movement, however, and can move back out the annulus at the site of implantation. This device extrusion can occur in over 25% of cases for some designs. While the effect of the complication is not life threatening, the response is another surgery to reposition or replace the PDR, or to remove it altogether and likely replace it with a total disc replacement or a fusion procedure. There is mounting evidence that the nucleus material left in the disc cavity, even after an exhaustive removal procedure, can push against even a well-positioned PDR and be the cause of many of the device extrusions. When a posterior approach is used for removal, the remaining nucleus material left behind can push against a PDR. While more of this material could be removed if the disc is accessed via a lateral or an anterior approach, current information indicates that most spine surgeons prefer to use the posterior approach.
The trend in TDR designs, like the trend in fusion procedures, is to minimize tissue trauma by reducing the invasiveness of the procedure. These devices typically rely on bony in-growth of the vertebrae-contacting portions of the device with the vertebral endplates to assure the devices remain in proper position following implantation. As with fusion procedures, any remaining nucleus will likely have a negative impact on the process of bony in-growth of the device and may lead to an increased incidence of device movement.
For intervertebral fusion, TDR and PDR, among other procedures, implantation site preparation typically involves removal of the nucleus. A wide range of devices have been developed for this removal procedure. However, surgeons have historically utilized an array of pituitary rongeurs and curettes for the various procedures requiring removal of the nucleus pulposus or portions of the nucleus pulposus. A rongeur is a single-hand “pistol-grip” actuated mechanical instrument with two cup-shaped hinged jaws that cut and rip tissue. The rongeur is provided in a variety of configurations including “up-biting”; straight; and “down-biting”, and can be found in a variety of lengths, widths, and with razor or serrated jaws. A curette is a rigid tool with a sharpened scraping tip that is often in the shape of a cup or ring, and also is provided in a variety of configurations. Using the preferred posterior access to the intervertebral disc with a rongeur or curette limits the useful range of motion within the disc. The bony structure of the posterior spinal elements, even though partially removed to provide access for implantation of the interbody device such as a fusion implant or PDR, typically limits the angles through which the rongeur or curette can be maneuvered. This limitation of movement serves to limit the amount of nucleus material that can be removed. More importantly, the limitation on movement may not allow adequate removal of material contralateral to the annular access, preventing optimal position for a PDR and inhibiting bone growth for a fusion procedure. Further, the use of a rongeur or curette requires constant insertion and removal to clean the nucleus material from the tip of the device, resulting in dozens of insertion/removal steps to remove an adequate amount of material from the nucleus. This can increase the trauma to the surrounding annulus tissue and increase the risk of damaging the endplates.
An additional significant limitation of both the rongeur and curette is the ability to easily remove the important annular tissue. Surgeons typically do not try to remove the entire nucleus in simple discectomy procedures, or intentionally remove annulus in preparation for fusion procedures; the annulus is intended to be preserved to help stabilize the spine as part of the treatment. Furthermore, most surgeons perform these procedures using tactile feedback to judge their position in the disc space and the type of material they are removing. These surgeons are “working blind” and rely on their experience and training to determine when to keep removing tissue or when to stop. In this respect, a surgeon's “feel” for the tissue, or ability to distinguish softer nucleus tissue from tougher annulus tissue, may not be well developed and PDR site preparation may result in significant trauma to the annulus. Important tissues surround the annulus, such as nerve roots that descend from the spinal column, the lumbar nerve plexus and major blood vessels including the aorta. Damaging these tissues can result in paralysis and death, and the risk of these complications is recognized as inherent in spine surgery.
In contrast to spinal fusion procedures, where the cartilage layer on the endplates of the vertebrae that contact the nucleus is removed along with the nucleus (to allow blood from the cancellous portion of the vertebrae to enter the disc space and provide the nutrients and proteins necessary for bone growth), nucleus replacement procedures seek to maintain the integrity of the cartilage layer to prevent the bone growth process from occurring. Rongeurs and curettes used to remove the nucleus have the capability to easily remove the cartilage, resulting in a potential for unwanted damage of the cartilage and subsequent growth of bone throughout the disc and vertebral fusion in a procedure where the intent is to maintain disc motion.
A range of more sophisticated devices for removing nucleus has been developed, however, the adoption of these devices has been very limited. Some of the more intricate devices utilize mechanized cutting mechanisms for removal of material from the nucleus pulposus. Frequently, these devices require suction and/or irrigation to remove material during the procedure.
One device uses a guillotine-style assembly that cuts nucleus material, aspirates the material into the instrument tip, and then evacuates the cut material is through the instrument. Movement of the guillotine assembly is automated and controlled by a mechanism in the handpiece of the instrument. The continuous removal of tissue without the need to repeatedly insert and remove the instrument minimizes trauma to the surrounding tissue. The guillotine type assembly is found on a straight, stiff device, that is intended for a minimally invasive, percutaneous approach. Because of their stiffness, although the devices may be somewhat effective for a lateral or anterior surgical approach for PDR implantation, they are generally not usable for nucleus removal utilizing a posterior approach and their small size prevents efficient removal of nucleus material from the entire disc cavity.
Other devices have utilized an Archimedes type screw to pull nucleus material into the catheter and shear it when it reaches the tip of the catheter. Continued collection of nucleus material by the rotating Archimedes type screw pushes the sheared material through the catheter and into a collection chamber. While less complicated to use than the previously discussed guillotine type assembly, the devices utilizing the Archimedes type screw typically have the similar maneuverability and bulk tissue removal disadvantages. Further, these devices can relatively easily be directed into and through the annulus of the intervertebral disc being treated.
Still other systems have used a high-pressure stream of water to remove nucleus material. In one device, the high-pressure stream of water produces a vacuum which pulls nucleus material into the stream. The high-pressure stream of water then cuts the nucleus material and pulls the material through a catheter to a collection bottle. Among other disadvantages, such systems are expensive. Further, the tip of the instrument can be bent only slightly since its design relies heavily on the use of a stiff metal tube to withstand the high pressure of the water stream, such that its lateral reach when used via the posterior approach is still very limited. Further, since the water stream is very narrow, successful nucleus removal can be technique dependent and time consuming.
Still other devices utilize radio frequency (RF) energy or plasma directed through electrodes for tissue resection and vessel cauterization in preparation for implanting a PDR. These devices typically include an RF generator that can be used with a variety of different types and shapes of electrodes. These devices are typically stiff and have little lateral reach when used making them relatively ineffective for use through the posterior approach. Further, the RF ablation technology can resect annulus or endplate cartilage as easily as nucleus material, as well as other critical nerve and vascular tissues surrounding the annulus.
Still other devices utilize lasers to remove material from the nucleus pulposus. These lasers are typically transmitted through a laser fiber positioned within a multi-lumen catheter. These multi-lumen catheters have also included additional components such as imaging fibers, illumination fibers, and irrigation ports. Further, the tip of these catheters can be slightly steerable. Although steerable, the bend radius of the catheters typically prevents them from being useful for removing nucleus near the annulus access and limits their reach into the area of the disc contralateral to the annular access. Further, the effective radius of laser beam from these devices is typically only 0.5 mm, making removal of large amounts of nucleus very difficult and time consuming. Detrimentally, lasers can resect annulus or endplate cartilage as easily as nucleus material. Since the tip of the catheter is typically not protected, the laser beam has the ability to easily penetrate and damage the annulus and endplate tissue, as well as other critical nerve and vascular tissues surrounding the annulus.
Other devices for nucleus removal are also available. However, these technologies possess their own limitations for the unique needs of annulus repair and PDR device site preparation. The limitations of these devices, along with those of the pituitary rongeur, are driving the need for a more advanced instrument for nucleus removal.
In the case of the spinal disc the three materials proximate to each other, the nucleus, annulus and cartilage, each have different biological constituents and mechanical properties. The nucleus is primarily made up of proteoglycans such as hyaluronic acid, a material that swells and is extremely slippery upon contact with water. In a degenerated disc being treated with a surgical procedure, the nucleus typically has experienced significant water loss even though the hyaluronic acid is still present and still able to absorb water. In hydrated form, the nucleus is extremely slippery and has been characterized as gelatinous; in dehydrated form it has adhesive qualities (is “sticky”) and has been likened in texture to “crab meat”. In either case, nucleus tissue is a relatively soft, mobile material. The annulus is a tough ligamentous structure comprised primarily of long collagen chains. Extreme degeneration can affect the toughness of the annulus, but it is always less elastic and far less mobile than the nucleus tissue. The nature of the annulus can be characterized as a reinforced rubber. While also containing collagen, the cartilage tissue is different from the nucleus and annulus and exhibits properties similar to a harder, polyethylene-like plastic material. The cartilage is the least mobile tissue of the three found in the disc space. The disparate properties of these materials, especially of the mobility of the nucleus tissue compared to the relative immobility of the annulus and cartilage, allows for an appropriately designed cutting instrument to take advantage of these differences for removal of the nucleus while leaving the annulus and cartilage undamaged.
Thus, current cutting instruments have many shortcomings including the shortcomings discussed above.